Natural products have long been recognized as a rich source of potent therapeutics but further development is often limited by high structural complexity and high molecular weight. In contrast, at the core of the thujaplicins is a lead-like tropolone scaffold characterized by relatively low molecular weight, ample sites for diversification, and metal-binding functionality poised for targeting a range of metalloenzyme drug targets. Here, we describe the development of this underutilized scaffold for the discovery of tropolone derivatives that function as isozyme-selective inhibitors of the validated anticancer drug target, histone deacetylase (HDAC). Several monosubstituted tropolones display remarkable levels of selectivity for HDAC2 and potently inhibit the growth of T-cell lymphocyte cell lines. The tropolones represent a new chemotype of isozyme-selective HDAC inhibitors. KEYWORDS: Tropolone, HDAC, isozyme-selectivity, thujaplicin, metalloenzyme, T-lymphocyte cancer cell lines N atural products have long served as a rich source of drugs for a variety of indications ranging from anticancer to antimicrobial to neurological disorders. Typically these natural products are characterized by high molecular weight and potency as well as high levels of structural complexity with limited sites for diversification. In contrast, thujaplicins, members of the tropolone family of natural products, can be regarded as lead-like natural products. β-Thujaplicin (also known as hinokitiol) is characterized by low molecular weight (MW = 164) and a relatively lower level of complexity that allows more extensive structural modification. Thujaplicins are monoterpene natural products isolated from the heartwood of trees in the Cupressaceae family 1 that are associated with antiproliferative activity.2−4 There have been few attempts to utilize these lead-like compounds in drug discovery, perhaps exacerbated by limited synthetic accessibility to these nonbenzenoid aromatics.The tropolone functionality is uniquely disposed to strongly chelate metal ions, which may be a hallmark of the biological activity of these compounds.3 Substituted tropolones are a compelling and distinct chemotype for the development of inhibitors of metalloenzyme drug targets. Herein, we describe our efforts to use β-thujaplicin as a lead-like natural product to develop a novel class of inhibitors of histone deacetylase, a validated target in the treatment of cancer. 5,6 Of the 18 HDAC isoforms, 11 are metalloenzymes that use zinc to remove a terminal acetyl group from lysine residues present in histones and other client proteins. The reversible acetylation and hydrolysis of the ε-acetamide in histones is associated with regulation of gene expression. Interestingly, there are a variety of natural products that inhibit HDACs such as trichostatin A (TSA; Figure 1), romidepsin, and trapoxin. Both romidepsin and vorinostat were approved by the FDA for the treatment of cutaneous T-cell lymphoma; the latter possesses a zinc-targeting hydroxamate, similar to TS...

The goal of many metabolomic studies is to identify the molecular structure of endogenous molecules that are differentially expressed among sampled or treatment groups. The identified compounds can then be used to gain an understanding of disease mechanisms. Unfortunately, despite recent advances in a variety of analytical techniques, small molecule (<1000 Da) identification remains difficult. Rarely can a chemical structure be determined from experimental “features” such as retention time, exact mass, and collision induced dissociation spectra. Thus, without knowing structure, biological significance remains obscure. In this study we explore an identification method in which the measured exact mass of an unknown is used to query available chemical databases to compile a list of candidate compounds. Predictions are made for the candidates using models of experimental features that have been measured for the unknown. The predicted values are used to filter the candidate list by eliminating compounds with predicted values substantially different from the unknown. The intent is to reduce the list of candidates to a reasonable number that can be obtained and measured for confirmation. To facilitate this exploration, we measured data and created models for two experimental features; MS Ecom50 (the energy in eV required to fragment 50% of a selected precursor ion) and HPLC retention index. Using a dataset of 52 compounds, Ecom50 models were developed based on both Molconn and CODESSA structural descriptors. These models gave r2 values of 0.89 to 0.94 depending on the number of inputs, the modeling algorithm chosen, and whether neutral or protonated structures were used. The retention index model was developed with 400 compounds using a back propagation artificial neural network and 33 Molconn structure descriptors. External validation gave a v2 = 0.86 and standard error of 38 retention index units. As a test of the validity of the filtering approach, the Ecom50 and retention index models, along with exact mass and collision induced dissociation spectra matching, were used to identify 1,3-dicyclohexylurea in human plasma. This compound was not previously known to exist in human biofluids and its elemental formula was identical to 315 other candidate compounds downloaded from PubChem. These results suggest that the use of Ecom50 and retention index predictive models can improve non-targeted metabolite structure identification using HPLC/MS derived structural features.

A generic activation mode for asymmetric LUMO-lowering catalysis has been developed using the long-established principles of oxy-allyl cation chemistry. Here, the enantioselective conversion of racemic α-tosyloxy ketones to optically enriched α-indolic carbonyls has been accomplished using a new amino alcohol catalyst in the presence of electron-rich indole nucleophiles. Kinetic studies reveal that the rate-determining step in this SN1 pathway is the catalyst-mediated α-tosyloxy ketone deprotonation step to form an enantiodiscriminant oxy-allyl cation prior to the stereodefining nucleophilic addition event.

The asymmetric total syntheses of the natural products (+)- and (-)-frondosin B and (+)-frondosin A are reported based on a diastereoselective cycloaddition between tetrabromocyclopropene and an annulated furan to provide a highly functionalized common building block. The bridged bicyclic intermediate could be stereo- and chemoselectively manipulated to produce the two structurally distinct members of the frondosins. Both syntheses feature regioselective palladium-coupling reactions and an unprecedented phosphine-mediated ether bridge cleavage. Surprisingly, the planned enantioselective synthesis of frondosin B led to the opposite epimer of the natural product, suggesting an unusual late stage stereoinversion at C8. Frondosin A, but not frondosin B, was shown to have selective antiproliferative activity against several B-cell lines.

A stereocontrolled approach to a key platensimycin intermediate was achieved from a commercially available furylcarboxylate. Key to our approach is the highly efficient formal [4 + 3] cyclocondensation of a substituted furan with tetrabromocyclopropene along with an intramolecular γ-alkylation to construct the final ring of the caged intermediate.